Yes, Medhead, that does work for individual flight requests.
But my post referred back to JessicaTam's suggestion for recording the preference in your Profile.
For some reason, it seems that can only be done initially by direct contact with QF:
https://qantas.custhelp.com/app/answers/detail/a_id/45
I won't attempt another quote, as this message has disappeared twice, so far, before I've clicked to post, but I recall reading that you can change a preference in your Profile once it's recorded. I can't even hazard a guess as to the reason for the two step process.